Patient Posture Determination and Stimulation Program Adjustment in an Implantable Stimulator Device Using Impedance Fingerprinting

ABSTRACT

Methods and circuitry for determining an implanted-neurostimulator patient&#39;s position, and adjusting a situation program delivered by the neurostimulator based on the determined position, is disclosed. Impedance measurements of the patient&#39;s tissue are taken at the neurostimulator&#39;s electrodes, which measurements can comprise complex impedance measurements (magnitude and phase) taken at different frequencies. Such impedance measurements, which can be taken interleaved with stimulation therapy, are used to determine an “impedance fingerprint.” This fingerprint can be compared to other known fingerprints stored in the IPG, which known fingerprints are associated with particular stimulation programs. When a measured fingerprint matches one stored in the IPG, the stimulation program associated with the stored fingerprint is automatically used for patient therapy. As different measured fingerprints are encountered, the IPG can learn and store a new stimulation program for such fingerprint by remembering stimulation parameters selected by the patient when such fingerprint is encountered.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/024,276, filed Sep. 11, 2013, which is a non-provisional of U.S.Provisional Patent Application Ser. No. 61/734,629, filed Dec. 7, 2012.Priority is claimed to these applications, and they are incorporatedherein by reference in their entireties.

FIELD OF THE INVENTION

The present invention relates to implantable stimulator devices, such asspinal cord stimulators.

BACKGROUND

Implantable stimulation devices are devices that generate and deliverelectrical stimuli to nerves and tissues for the therapy of variousbiological disorders, such as pacemakers to treat cardiac arrhythmia,defibrillators to treat cardiac fibrillation, cochlear stimulators totreat deafness, retinal stimulators to treat blindness, musclestimulators to produce coordinated limb movement, spinal cordstimulators to treat chronic pain, cortical and deep brain stimulatorsto treat motor and psychological disorders, and other neural stimulatorsto treat urinary incontinence, sleep apnea, shoulder subluxation, etc.The description that follows will generally focus on the use of theinvention within a Spinal Cord Stimulation (SC S) system to treat lowerback pain, such as that disclosed in U.S. Pat. No. 6,516,227. However,the present invention may find applicability in any implantablestimulator device

As shown in FIGS. 1A-1C, a SCS system typically includes an ImplantablePulse Generator (IPG) 10, which includes a biocompatible device case 12formed of a conductive material such as titanium for example. The case12 typically holds the circuitry and battery 14 (FIG. 2B) necessary forthe IPG to function, although IPGs can also be powered via external RFenergy and without a battery. The IPG 10 is coupled to electrodes 16 viaone or more electrode leads 18, such that the electrodes 16 form anelectrode array. The electrodes 16 are carried on a flexible body 20,which also houses the individual signal wires 22 coupled to eachelectrode. In the illustrated embodiment, there are eight electrodes onlead 18, labeled E₁-E₈. However, the number of electrodes on a lead, aswell as the number of leads, are application specific and therefore canvary. The lead 18 couples to the IPG 10 using a lead connector 24, whichis fixed in a non-conductive header material 26, which can comprise anepoxy for example.

As shown in the cross-section of FIG. 2B, the IPG 10 typically includesan electronic substrate assembly including a printed circuit board (PCB)30 containing various electronic components 32. Two coils (moregenerally, antennas) are generally present in the IPG 10: a telemetrycoil 34 for transmitting/receiving data to/from an external controller50; and a charging coil 36 for charging or recharging the IPG's battery14 using an external charger (not shown). (FIG. 1B shows the IPG 10 withthe case 12 removed to ease the viewing of the two coils 34 and 36).

FIG. 2A shows a plan view of the external controller 50, and FIG. 2Bshows the external controller 50 in relation to the IPG 100 with whichit communicates. The external controller 50 is shown as a traditionalhand-held patient controller, although it could also comprise aclinician programmer of the type typically used in a clinician's office.(A clinician external controller would look differently, as one skilledin the art understands, and typically comprises computer). The externalcontroller 50 is used to send data to and receive data from the IPG 10.For example, the external controller 50 can send programming data suchas therapy settings to the IPG 10 to dictate the therapy the IPG 10 willprovide to the patient. Also, the external controller 50 can act as areceiver of data from the IPG 10, such as various data reporting on theIPG's status.

As shown in FIG. 2B, the external controller 50, like the IPG 100, alsocontains a PCB 52 on which electronic components 54 are placed tocontrol operation of the external controller 50. The external controller50 is powered by a battery 56, but could also be powered by plugging itinto a wall outlet for example.

The external controller 50 typically comprises a user interface 60similar to that used for a portable computer, cell phone, or other handheld electronic device. The user interface 60 typically comprisestouchable buttons 62 and a display 64, which allows the patient orclinician to send therapy programs to the IPG 10, and to review anyrelevant status information reported from the IPG 10.

Wireless data transfer between the IPG 10 and the external controller 50preferably takes place via inductive coupling. This typically occursusing a well-known Frequency Shift Keying (FSK) protocol, in which logic‘0’ bits are modulated at a first frequency (e.g., 121 kHz), and logic‘1’ bits are modulated at a second frequency (e.g., 129 kHz). Toimplement such communications, both the IPG 10 and the externalcontroller 50 have communication coils 34 and 58 respectively. Eithercoil can act as the transmitter or the receiver, thus allowing fortwo-way communication between the two devices. This means ofcommunicating by inductive coupling is transcutaneous, meaning it canoccur through the patient's tissue 70.

The lead 18 in an SCS application is typically inserted into theepidural space 80 proximate to the dura 82 within the patient's spinalcord, as illustrated in cross section in FIG. 3. The proximate portionof the lead 18 is tunneled through the patient where it is attached tothe lead connector 24 of the IPG 10, which is implanted a somewhatdistant location from the lead, such as in the upper portion of thepatient's buttocks. Typically in an SCS application, there are two leadsimplanted in the left and right sides of the spinal column, so thatstimulation therapy can be delivered by the IPG 10 to left- andright-branching nerves from the dura 82. However, only one such lead isshown in FIG. 3 for simplicity.

Once implanted, the patient is typically put though a fitting procedureto determine effective therapy to treat the patient's symptoms. Thistypically occurs in a clinician's office, and may be somewhatexperimental in nature. (Some aspects of fitting may also occur prior tofull implantation of the IPG 10 during an external trial stimulationphase, which is discussed in U.S. Patent Publication 2010/0228324 forexample).

Generally speaking, during the fitting procedure, various stimulationparameters are applied by the IPG 10 to determine what feels best forthe patient, and then such stimulation parameters can then be stored inthe IPG 10 as a stimulation program. Stimulation parameters can includewhich electrodes 16 on the lead 18 are active, the polarity of theactive electrodes (i.e., whether they act as anodes (current sources) orcathodes (current sinks)), the magnitude of the current pulses appliedat the active electrodes (which may comprise either a voltage or currentmagnitude), the duration of the pulses, the frequency of the pulses, andother parameters. These stimulation parameters can be varied by theexternal controller 50—either a clinician programmer or a hand-heldpatient controller—which wirelessly communicates with the telemetry coil34 in the IPG 10 to change and store the parameters in the IPG 10. Aftera stimulation program has been set by the clinician, and the patient hasleft the clinician's office, the patient can modify the stimulationparameters of that program using his hand-held patient controller.

The art has recognized that patients may benefit from the use ofdifferent stimulation at different times, and in particular depending onthe patient's posture or activity. This is because the position of thelead 18 may move in the epidural space 50 as the patient changes moves,e.g., from supine (on one's back), to standing, to prone (on one'sstomach). This is shown by the arrows in FIG. 3. As seen, as the patientmoves, the distance between the electrodes 16 and the dura 82 canchange. This change in distance can warrant changes in therapy.

In the prior art, such changes in patient posture or activity weresensed by an accelerometer in the IPG 10. As is well known, anaccelerometer can detect its position in three-dimensional (3D) space byassessing gravitational forces, and so can detect the 3D position of apatient in which it is implanted. According to this technique, a patientis somewhat relieved of the obligation to manually adjust hisstimulation parameters when changing postures, because the IPG canrecognize a change in body posture and remember the level of stimulationneeded. Thus, in the prior art, an IPG 100 could sense when the patientchanges posture; learn from previous experience and remember thepatient's last comfortable setting for that posture; and respond byautomatically adjusting stimulation to the patient's chosen setting forthat posture.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B show different views of an implantable medical device,specifically an Implantable Pulse Generator (IPG).

FIGS. 2A and 2B show an external controller for communicating with theIPG.

FIGS. 3, 4A, and 4B show an electrode leads positioned in an epiduralspace within a patient's spinal cord, and shows how the positioning ofthe lead can change based on a patient's posture.

FIG. 5 shows an illustration of impedance monitoring circuitry inaccordance with an embodiment of the invention for measuring animpedance between two electrodes of an IPG, including an impedancealgorithm.

FIG. 6 shows how the impedance algorithm can compile the impedancemeasurements into an impedance fingerprint indicative of the tissueimpedance between various electrode combinations.

FIG. 7 shows an example of how the impedance measurements in FIG. 6 fora given electrode combination can be taken at more than one frequency.

FIG. 8 shows how the impedance algorithm interfaces with a fingerprintdatabase comprising known impedance fingerprints and associatedstimulation programs indicative of a patient posture, and interfaceswith a fingerprint log comprising measured fingerprints.

FIGS. 9A and 9B show a display of an external controller incommunication with an IPG comprising the impedance monitoring circuitry,and show how a user can review and rename postures identified by theIPG, and modify the stimulation program associated with an identifiedposture.

FIGS. 10A and 10B show how and when the impedance measurementscomprising an impedance fingerprint can be taken during operation of astimulation program.

DETAILED DESCRIPTION

The inventors see shortcomings from the prior-art-based method of usingan accelerometer in an IPG to sense patient posture and thus todetermine an appropriate stimulation program for the patient. For one,the prior art does not consider that forces other than accelerative(e.g., gravitational) forces might be impingent upon the patient in amanner indicative of posture and thus affecting therapy. This can leadto misleading assumptions about the patient's posture. For example, anaccelerometer in an IPG might conclude that a patient lying prone and apatient standing but bending forward are in the same posture, whenreality is otherwise. Likewise, an accelerometer might conclude that apatient standing up and a patient sitting down are in the same posture,again when this is not true.

The inventors see such distinctions as significant, particularly uponrealizing that the positioning of electrodes on the lead 18 can varyrelative to the dura 82 in unpredictable and non-uniform ways. ConsiderFIGS. 4A and 4B. In FIG. 4A, it is assumed that the patient is lyingprone, with the result that the lead 18 has generally moved away fromthe dura 82 by a constant distance, d, along its entire length. Bycontrast, in FIG. 4B, it is assumed the patient is standing but bendingforward. When the patient bends in this manner, the epidural space 80and dura 82 will also bend while the lead 18 remains relativelystraight, with the result that the distance between the electrodes andthe dura 80 is not constant, with some electrodes closer and somefarther away. Additionally, when the patient bends, the lead 18 can alsoshift laterally in the epidural space 80, as shown by the arrow in FIG.4B. This too changes the positioning of the electrodes relative to thedura 50. For example, consider a nerve branch 84 that causes pain for apatient and requires neurostimulation. In FIG. 4A, activation ofelectrode E2 would be most likely to recruit and treat nerve branch 84.However lateral shifting of the lead in FIG. 4B now brings electrode E1closer to nerve branch 84, suggesting that activation of that electrode,rather than E2, would provide the best therapy for the patient.

Despite these postural differences, an accelerometer based approachmight determine that both of the patient postures in FIGS. 4A and 4B arethe same, and therefore might conclude that a single stimulation programis appropriate for both postures, when in reality, different stimulationprograms for each posture may be beneficial.

In recognition of this fact, the inventors approach to determiningpatient posture, and hence what stimulation program is effective, relieson measurements taken at the electrodes. Specifically, and as will bediscussed in further detail below, impedance measurements of thepatient's tissue are taken at the electrodes, which measurements cancomprise complex impedance measurements (magnitude and phase) taken atdifferent frequencies. Such impedance measurements, which can be takeninterleaved with stimulation therapy, are used to determine an“impedance fingerprint.” This fingerprint can be compared to other knownfingerprints stored in the IPG, which known fingerprints are associatedwith particular stimulation programs. When a measured fingerprintmatches one stored in the IPG, the stimulation program associated withthe stored fingerprint is automatically selected for patient therapy. Ifthe stimulation program is changed for a given posture, such changeswill be stored with its associated fingerprint to remember these changesfor the next time the patient is in the relevant posture. Additionally,as new fingerprints are measured which do not match those stored, theIPG can learn and store a new stimulation program for such newfingerprint by remembering stimulation parameters selected by thepatient when such fingerprint is encountered. As such, the IPG can learnnew postures and new stimulation programs “on the fly” as simulation isoccurring, and as the patient goes about his day.

Before discussing particulars of the technique, an example of impedancemonitoring circuitry 100 useable in an IPG 10 in accordance with thetechnique is disclosed in FIG. 5. For simplicity, the circuitry 100 isshown for only a single electrode E, but such circuitry would in anactual implementation be duplicated or shared so that the impedance atall electrodes can be monitored. The depicted impedance monitoringcircuitry is but one example of circuitry capable of performingimpedance measurements, and should not be taken in a limiting sense.

Electrode E is shown relative to another reference electrode, Ref, which(as will be discussed subsequently) could comprise another electrode onthe lead 18 or the IPG's conductive case 12 (FIG. 1A). A patient'stissue intervening between the electrode and the reference is modeled102 as an R-C network, with a capacitor C and resistor R1 in parallel,and that parallel combination coupled to a series resistor R2. Generallyspeaking, tissue fluids are modeled by R2, while cell membranes in thetissue are modeled by the parallel connection of C and R1. Tissue model102 could be further refined, with different resistances andcapacitances to better fit the reality of the environment of the tissue,but the simple three component model 102 serves for purpose ofillustration. By virtue of the capacitance, the impedance Z of thetissue will be complex, as shown by the equations in FIG. 5. As such,the AC voltage V produced across the tissue in response to a constant ACcurrent through the tissue will be phased-shifted (0) relative to thecurrent, as shown in the waveforms.

In one example, the measurement signal for the impedance measurement isprovided by a AC constant current source 105. The magnitude, andfrequency, f, of the produced current is controlled by an impedancealgorithm 120 operating in a microcontroller 110, which algorithm willbe discussed in further detail below. Microcontroller 110 can compriseany known central processor or controller typically present in an IPG100, or any logic circuitry more generally. The voltage drop across thecurrent source can be monitored by a compliance voltage regulator 115,which is used to generate a compliance voltage V+ sufficient to providethe desired current I without loading, and in an efficient mannerconsiderate of power draw from the IPG's battery 14. Suitable compliancevoltage regulation circuitry 115 can be found in U.S. Patent ApplicationSer. No. 61/654,606, filed Jun. 1, 2012, which is incorporated byreference, and with which the reader is assumed familiar. Typically, themagnitude of the constant current used for the impedance measurementwill be much lower than typical magnitudes used for therapeuticcurrents, and hence won't be noticed by the patient. For example, canrange from 1 microamp to 100 microamps. Current source 105, depending onits complexity, can also be used to generate the therapeutic stimulationpulses to the patient (see FIG. 10), or may be separate from the sourcesused to generate the stimulation pulses.

Monitoring of tissue impedance occurs under the control of the impedancealgorithm 120 operating in the microcontroller 110, which algorithm, aswill be discussed further later, can operate while the IPG 10 isproviding stimulation to the patient. At an appropriate time for ameasurement, the impedance algorithm 120 sends the magnitude, andfrequency, f, of the measuring signal Ito the current source 105. Theresulting AC voltage V at electrode E resulting in response to themeasurement signal I is then digitized at an Analog-to-Digital (A/D)converter 125, which samples the voltage at an appropriate rate (such asevery ten degrees) and an appropriate number of times (such as over 180degrees) to allow the impedance algorithm 120 to discern the waveform'smagnitude, |V|, and phase, 0, relative to the current. (It is assumedhere that the impedance algorithm 120 understands the relative phase 0between the voltage and the current by virtue of its control of thecurrent source 105. However, if necessary, the phase of the current Ican also be monitored, and compared with the voltage at a phasedetector, to provide the relative phase 0 to the impedance algorithm).

Once the voltage magnitude, |V|, and phase, 0, have been determined bythe impedance algorithm 75, the complex impedance, Z, of the tissue canbe calculated based upon the impedance algorithm 75's a priori knowledgeof the current magnitude, Specifically, and as shown in the equations inFIG. 5:

Z=V/I=|V|/|I|*e ^(−jθ) =|Z|*e ^(−jθ)

where the magnitude of the impedance, |Z|, comprises the ratio betweenthe magnitudes of the voltage and the current (i.e., |Z|=|V|/|I|). Themagnitude, |Z|, and phase, θ, of this impedance 130 are stored by theimpedance algorithm 120 a given frequency. Such storage may be withinmemory in the microcontroller 110, or a separate memory associated withand controlled by the microcontroller. In a preferred embodiment, theimpedance measurement 130 can be repeated at electrode E at otherfrequencies, thus allowing new magnitudes, |Z|, and phases, θ, to bedetermined and stored, as discussed further below.

FIG. 6 illustrates how such impedance measurements 130 can be compiledand processed by the impedance algorithm 120 to determine an impedancefingerprint 150 of the tissue. Different electrodes may act as thereference electrode (Ref; FIG. 5) for any particular electrode whoseimpedance is being measured. For example, in electrode combinations140a, the conductive case 12 acts as the reference electrode (e.g.,ground) relative to each of the measured electrodes. This is desirableto understand each electrode's impedance relative to something akin to asystem ground, which a large case electrode essentially provides in therelatively conductive environment of a patient's tissue.

In electrode combinations 140b, the impedance at each electrode ismeasured relative to its neighbor: E2 to E1, E3 to E2, etc., and so suchneighbor acts as the reference. This is desirable because the electrodescan have different distances to relevant structures. For example, andreferring again to FIG. 4B in which the tissue is bent relative to thelead 18, if the impedances of the epidural space 80 and the dura 82differ, then it would be expected that the impedance between E4 and E3(which are relatively close to the dura 82) and electrodes E8 and E7(which are relatively far from the dura), would be significantlydifferent. In short, the combined information provided by electrodecombinations 140 a and 140 b should provide a reliable assessment of thetissue impedance relative to the electrodes. However, other electrodecombinations are possible, and greater numbers of electrode combinationimpedance measurements will increase the resolution of the resultingfingerprint, and thus increase the likelihood of eventuallyautomatically choosing an appropriate stimulation program based onpostural changes. Moreover, an electrode combination 140 may alsoinclude more than simply a pair of two electrodes, and can insteadcomprise an impedance measurement taken between a more-complex tissuenetwork defined by three or more electrodes.

Taking measurements at different frequencies allows the impedancealgorithm 120 to compute values for the various components between thetwo electrodes in the tissue model 102—R1, R2, and C—to be determined.Because tissue model 102 contains three unknown values, measurements aretaken at at least three frequencies, thus rendering a system ofequations from which the impedance algorithm 120 can deduce R1, R2, andC between a given electrode combination 140.

FIG. 7 shows a simple example of three such measurements, taken at lowintermediate, and high frequencies (f_(low), f_(int), and flue), andassuming that no decoupling capacitors 103 (FIG. 5) are included in themeasurement. (Decoupling capacitors 103 are discussed further below).The simulation assumes that R2=100 ohms, R1=900 ohms, and C=1 μF,although experimentation may be required to determine values that aremore indicative of a patient's actual tissue. If the frequency off_(int), is small enough (e.g., tens of Hertz), the capacitor C can beassumed to be an open circuit in the tissue model 102, meaning themeasured impedance magnitude (|Z|_(low)) is dominated by the sum of R1and R2. Likewise, if the frequency of f_(int) is high enough (e.g., tensof kilohertz), the capacitor C can be assumed to be a short circuit,meaning the measured impedance magnitude (|Z|_(int)) is dominated by R2alone. In other words, the values for R1 and R2 can be determined, or atleast estimated, solely using the impedance magnitudes at these twofrequencies. A third impedance measurement, f_(int), is taken at someintermediate frequency, which may be one which experimentation teachesresults in an impedance magnitude that is about half of the maximummagnitude expected (e.g., ½(R1+R2)). This third measurement renders animpedance of |Z|_(int) and θ_(int), which can then be used to determinethe value of the capacitor C. Specifically, and using the equations setforth in FIG. 5:

|Z| _(int) *e ^(−jθint) =R2+(R1/(1+R1*Cj2πf _(int)))

which can be solved for C. However, this simple method, whereby R1 andR2 are determined by making simple assumptions about the capacitance, isnot strictly necessary. Measurements at any three frequencies can beused, rendering a system of equations from which R1, R2, and C can besolved from (|Z|1, θ1, f1), (|Z|2, θ2, f2) and (|Z|3, θ3, f3).

While the particular tissue model 102 here assumes three unknown values,other more or less complex tissue models 102 could exist, warrantingdifferent numbers of minimal frequency measurements to allow for thevalues in such models to be solved. Inclusion of measurements at morethan a minimal number of frequencies allows the unknown tissue modelvalues to be determined with increased reliability. In one example, theimpedance measurements can be taken over frequencies ranging from about20 Hz to 50 kHz.

It is known in the art of implantable neurostimulators to connectdecoupling capacitors 103 (FIG. 5) to stimulating electrodes to preventthe direct injection of DC current into a patient tissue, which can be asafety concern. If present, such decoupling capacitors 103 may beincluded in the impedance measurement, yielding a total measuredimpedance of

51 Z|*e ^(−jθ)=(2/C _(de) j2πf)+R2+(R1/(1+R1*Cj2πf)0

where (2/C_(de)j2πf) comprises the series impedance of the twodecoupling capacitors 103. Because the capacitances of the decouplingcapacitors, Cde, are known, they will not affect the ability to discernthe unknown tissue values of R1, R2, and C by measuring at threedifferent frequencies. (Note however that the inclusion of thedecoupling capacitors 103 in the impedance measurement does not allowfor the simple approximation of R1 and R2 at high and low frequencies,as shown in FIG. 7).

Referring again to FIG. 6, once the impedance algorithm 120 has computedthe tissue model values (R1, R2, and C) for each of the electrodecombinations 140, these values taken together comprise an impedancefingerprint (FP) 150. While it is informative that the fingerprint 150comprises the solved-for values of electrical components in the tissuemodel 102, this is not strictly necessary. Instead, the fingerprint 150can be comprised of any other suitable metric or metrics indicative ofthe complex impedance of the various electrode combinations 140. Forexample, the portion of the impedance fingerprint 150 for any givenelectrode combination 140 can comprise the raw impedance magnitudes andphases (|Z≡1, θ) measured at the various frequencies, or some otherprocessing of this raw information short of solving tissue model values.Indeed, because it is not strictly necessary to solve tissue modelvalues, it is not strictly necessary that the fingerprint 150 compriseinformation taken at any particular number of frequencies. Experiencemay teach that complex impedances measured at a single frequency may besufficient to reliably discern patient posture. Additionally, theimpedance fingerprint 150 may involve processing the impedancemeasurements or tissue model values for each of the electrodecombinations together into a new metric.

The impedance fingerprint 150 is stored by the impedance algorithm 120in a fingerprint database 200, as shown in FIG. 8. The fingerprintdatabase 200, like the impedance measurements 130, can be stored in anysuitable memory in the IPG 10. The fingerprints 150 in the database 200are each associated with a stimulation program (SP) 160, which may bedetermined during a fitting procedure with a clinician, or which may benew fingerprints learned on the fly, as discussed below.

Stimulation programs SPa-SPf are assumed in the illustrated example ofFIG. 8 to be determined during a fitting procedure. For example, theclinician will ask the patient to stand, and then instruct the IPG 100using an external controller 50 to take and store an impedancefingerprint 150 (FPa). While still standing, the clinician will adjustthe stimulation parameters using the external controller 50—which againmay comprise one of more of which electrodes are active, the polarity ofthe active electrodes, the magnitude, duration, and frequency of thecurrent pulses applied at the active electrodes, or other parameters—todetermine a stimulation program 160 that provides desirable therapy forthe patient in the standing position (SPa). The fingerprint (FPa) andstimulation program (SPa) are then associated in the fingerprintdatabase 200. Thereafter, the clinician will have the patient take newpostures, and repeat the above procedure to determine variousfingerprints (FPx) and stimulation programs (SPx) and store andassociate them in the fingerprint database 200.

It is not strictly necessary that the fingerprint database 200 bepre-populated with posture related fingerprints 150 and associatedstimulation programs 160 during a fitting procedure, as the system canlearn and store new fingerprints and stimulation programs on the fly, asdiscussed further below. Both learned fingerprints and fingerprintsdetermined during a fitting session are said to be pre-determined oncestored in the database 200.

It should be noted that the fingerprints 150 stored in fingerprintdatabase 200 may from time to time require updating. Such updating isuseful when it is recognized that the physiology around the IPG 10 andlead 18 can change over the course of its useful life. For example, scartissue may develop around the electrodes 16 on lead 18 or the case 12,or the lead 18 may over time settle into a particular position in theepidural space 80, either of which may require altering of thefingerprints 150 in the database 200. As such, it should be understoodthat the fingerprints 150 in database 200 will from time to time beupdated, either in a clinician's office as described above, or under thecontrol of the patient. For example, the microcontroller 110 in the IPG10 can be controlled to communicate to the external controller 50 whenit is reasonable to update the various fingerprints stored in thedatabase 200, perhaps every six months or so. When the patient is sonotified by the external controller 50, the user interface 60 of theexternal programmer 50 can be programmed to walk the patient through afitting procedure similar to that described above, allowing newimpedance measurements to be taken in the different postures understoodby the database 200, and to update the stored fingerprints 150associated with these postures. However, and again, such periodicre-fitting and pre-population of the fingerprint database 200 is notstrictly necessary if fingerprints are learned and stored in thedatabase 200 on the fly, as discussed further below.

The database 200 may optionally store a textual description 170 of theposture, which again may be entered into the external controller 50 bythe clinician during the fitting procedure. This can be useful to allowthe external controller 50 to query the various postures stored in thefingerprint database 200. For example, by communicating the textualdescription 170 of the currently-running stimulation program 160 to theexternal controller 50, the patient can verify that that program matchesthe patient's actual posture. Displaying a textual description 170 ofthe posture at the external controller 50 is also useful to allow thepatient to modify that stimulation program, or to select a differentstimulation program, as shown in example display 64 of the externalcontroller 50 in FIG. 9A. In a preferred embodiment, if the patientmanipulates user interface 60 of the external controller 50 to modifyany of the simulation parameters for a stimulation program 160 currentlyoperating in the IPG 10, as identified by user selection 66, thestimulation program 160 in the fingerprint database 200 will beautomatically updated with the new parameters so that these newparameters will be applied when the IPG 10 senses the fingerprint 150associated with that stimulation program 160. This allows the IPG 10 tolearn new parameters for the stimulation programs based on patientfeedback.

Once the fingerprint database 200 has been populated or updated in thismanner, the impedance algorithm 120 need merely take fingerprintmeasurements from time to time, match them to those stored in thedatabase 200 if possible, and choose the associated stimulation program160 from the database 200 to provide patient therapy. In this regard,and as shown in FIG. 8, measured fingerprints 180 can be stored in afingerprint log 210 associated with a timestamp, and (optionally) thestimulation program 190 running at that time. Log 210 as before can bestored in any suitable memory in the IPG 10. Although only strictlynecessary to the disclosed technique to store the currently-measuredfingerprint 180, storing a time-stamped history of fingerprints in thelog 210 will allow the impedance algorithm 120 to perhaps identify newpostures currently unknown to the database 200, as explained furtherbelow. The log 210 may be limited in its capacity to store only areasonable number of fingerprint measurements 180, for example, thoseoccurring over the course of a day.

At a given time (e.g., t3), the impedance algorithm 120 determines thecurrently-measured fingerprint 180 in the log 210 (e.g., FPc). If theimpedance algorithm 120 can match this measured fingerprint 180 to afingerprint 150 stored in the database 200 (e.g., FPc), the impedancealgorithm 120 will cause stimulation program SPc associated with FPc inthe database 200 to be applied to the patient. If the impedancealgorithm 120 cannot match the measured fingerprint 180 to a fingerprint150 in the database 200, the impedance algorithm 120 may take no actionwith regard to modifying the therapy currently being applied to thepatient. Or, the unrecognized measured fingerprint 180, once learned onthe fly, can be stored in the database 200 and associated with anappropriate stimulation program, as described in the next paragraph. Oneskilled in the art will understand that matching the measuredfingerprints 180 in log 210 with the stored fingerprints 150 in database200 may require a comparison that is acceptable within some margin oferror, as it cannot be expected that any given measured fingerprint 180will exactly match one stored in the database 200. For example, it maybe sufficient that all values (R1, R2, C) in the two fingerprints 180and 150 match within 20%, or that 85% of them match within 10%, or suchother statistical metrics that may be suitable based upon routineexperimentation and empirical data.

Other postures in the fingerprint database 200, such as “posture 1,” maypresently be unknown, but may be determined based on new impedancefingerprints that the IPG 100 learns on the fly. The fingerprint log 210previously discussed is useful in this regard. The impedance algorithm120 can from time to time check the fingerprint measurements 180 in thelog 210 to see if a particular fingerprint is occurring with relevantfrequency. For example, suppose a particular patient likes to sleep onhis right side—a posture that was not addressed during the fittingprocedure. The impedance algorithm 120 may notice a measured fingerprint180 (e.g., FPg) associated with this posture in log 210 that occursfrequently, but is unknown to the impedance algorithm 120 because itdoesn't match with any of the fingerprints 150 stored in the database200. When assessing measured fingerprints 180 to determine the frequencyof their occurrence, it cannot be assumed that any two (or more)measured fingerprints 180 in log 210 will match exactly, and again theimpedance algorithm 120 may need to employ statistical metrics to makethis determination.

When a predictable but unknown fingerprint (e.g., FPg) is present in log210, the impedance algorithm 120 may determine a new posture, and canadd this new fingerprint (perhaps as averaged where it variously occursin log 180) to the database 200. If this measured fingerprint 180 isassociated with stimulation program(s) 190 in the log 210, whichstimulation programs may have resulted from patient changes, someindication of the associated stimulation program(s) 190 can be stored inthe database 200 as well.

For example, if the stimulation programs associated with FPg are notuniform in the log 210 (e.g., SPw-SPz), the impedance algorithm 120 canaverage them in some manner to determine and store a singularstimulation program (SPg) with FPg in the database 200. Or, theimpedance algorithm 120 may simply store the last stimulation program inthe log 210 associated with FPg in the database 200 (e.g., SPz, whichmay simply default to the preceding stimulation program SPd). If reviewof the log 210 results in no important information relevant tostimulation associated with the new fingerprint—for example, if SPw-z donot indicate some change by the patient beyond the last stimulationprogram that was applied when the new fingerprint was encountered—thenSPg can simply be associated with default or currently-pendingstimulation parameters in the database 200. Once entered in the database200, the stimulation program SPg associated with new fingerprint FPg canbe updated based on the patient's modification of the stimulationparameters while in such posture, i.e., while FPg is currently beingapplied by the IPG 10, as explained earlier.

Thereafter, once new fingerprint FPg is recognized and learned by theimpedance algorithm 120 in the log 210 and stored in database 200, theIPG 10 can automatically switch therapy to its associated program (SPg)in the database 200 when this now pre-determined fingerprint isencountered, even though the IPG 10 has no a priori understanding of thepatient's posture based upon fitting. If textual descriptions 170 ofposture are provided in the database 200, the IPG 10 may assign ageneric textual description to such new and unidentified posture (i.e.,“posture 1”).

At some point in future, the IPG 10 can attempt to communicate with theexternal controller 50 to inform the patient that a new posture has beenidentified, and allow the patient to name this new posture. This isshown in FIG. 9B. As shown, the display 64 of the external controller 50identifies that the current posture of the patient is genericallydescribed as “posture 1,” and allows the patient to rename thedescription of this posture to something meaningful to the patient peruser selection 67, for example “right side.” If this textual descriptionis updated by the patient, the external controller 50 would telemeterthis new textual description 170 to database 200 to overwrite the“posture 1” placeholder provisionally assigned in the database 200 forthe relevant stored fingerprint 150 (FPg). If the patient does notchoose to update the textual description per selection 67, there is noconsequence to patient therapy: the impedance algorithm 120 can stillchange the stimulation program based on its detection of fingerprintFPg, including any patient modifications captured within stimulationprogram 160 with which it is associated.

The above example by which an unknown fingerprint FPg is determined andultimately associated with a stimulation program SPg in the fingerprintdatabase 200 illustrates that the fitting procedure described earlier isnot strictly necessary—that is, it is not necessary to pre-populate thedatabase 200 with fingerprint taken at known postures and associatedstimulation programs taken at known patient postures. Just as sleepingon the patient's right side was initially unknown to the IPG 10, all ofthe postures represented in FIG. 8 may be unknown but eventuallydiscovered. For example, the impedance algorithm may notice that afingerprint associated with a standing patient occurs frequently (FPa),and over time—and based on patient adjustment of the stimulationparameters while in that posture—can eventually learn and store thestimulation program (SPa) to associate with that fingerprint in thedatabase 200. Thus, the impedance algorithm 120 can entirelyself-populate the fingerprint database 200 thus allowing the IPG 10 tolearn which stimulation programs best suit the patient for a givenmeasured impedance fingerprint.

Even if physiological changes (scarring) eventually cause changes to thelearned fingerprints, such changes can be tracked and updatedautomatically. For example, an initial fingerprint FPa associated with astanding position may over time change, in which case the impedancealgorithm 120 will recognize the change, and may eventually determine anew fingerprint (e.g., FPh) for this position, which is stored in thefingerprint database 200 and associated with its stimulation program(e.g., SPh). The earlier fingerprint FPa for this position may thus overtime become moot, in which case it will never match thecurrently-measured fingerprints, and hence its associated stimulationprogram SPa will never be chosen by the impedance algorithm 120 forpatient therapy. In this regard, the impedance algorithm 120 can monitorhow often fingerprints in the database 200 are matched with thosecurrently being measured, and if particular fingerprints have not beenmatched for some time, the impedance algorithm may eventually deletesuch now-moot fingerprints and their associated stimulation programsfrom the database 200.

The disclosed technique is further beneficial because its reliance onelectrical measurements means that therapy can be adjusted as a resultof any factor affecting electrode impedance, whether based upon thepatient's posture or not. In this regard, “posture” should be understoodas including anything that can affect the positioning or impedance ofelectrodes in the patient, including static postures, active postures(such as walking or running), gravity, accelerative forces, etc.

The tissue impedance measurements 130 (FIG. 6) upon which the disclosedtechnique are based are preferably taken during the provision of astimulation program to the patient, and FIGS. 10A and 10B shows variousways in which this can occur. Shown are the current pulses being appliedto a particular electrode, which comprises an active electrode in thestimulation program currently operating. One skilled will realize thatsome other electrode will act as the return path for these currentpulses, but this is not shown for simplicity. Also, more than twoelectrodes may be active to provide a more complex therapy to thepatient, but again this is not shown.

The therapeutic stimulation pulses in this example are bi-phasic,meaning that they have portions of opposite polarities. This is commonin the neurostimulation art to assist in charge recovery from thetissue. To further assist in such recovery, the pulses are followed by apassive recovery period. Biphasic pulses and recovery periods areexplained in further detail in U.S Provisional Patent Applicant Ser. No.61/654,603, which with the reader is assumed familiar.

The recovery period is followed by a quiet period where no electrode isproviding therapy to the patient. These quiet periods are logical timesto take the impedance measurements 130 described earlier, which canoccur in different ways. Whether impedance measurements can be takenduring the quiet periods without interrupting or delaying the pulsesdepends on the duration of such quiet periods, which in turn depends onthe frequency, f_(pulse), of the pulses, and the duration of the pulsesand the recovery periods. Whether impedance measurements can be takenduring the quiet periods without interrupting or delaying the pulsesalso depends on the frequency f at which the impedance measurement istaken. Because a given impedance measurement may requiring sampling theresulting voltage V acorns the tissue for some significant portion ofits cycle (e.g., 180 degree as noted earlier), the measurement can betaken without interruption if this 180-degree timeframe can fit withinthe duration of the quiet period—i.e., if 1/(2f)≦t_(quiet)—a conditionthat is more easily met when the frequency of the impedance measurementis higher. If this condition is met, an impedance measurement 130× forat least one electrode combination 140 can be taken during each quietperiod, as shown in FIG. 10A.

In the example discussed earlier in an IPG 10 with eight electrodes, 45impedance measurements 130 would need to be taken to comprise a fullfingerprint 150 for each of the fifteen electrode combinations 140(eight from each electrode to the case, and seven between nearestneighbors) taken at three frequencies. Such measurements will generallynot be difficult to take in a reasonable amount of time for updating thefingerprints 180 in the log 210, which need occur only every minute orlonger. For example, for pulses occurring at f_(pulse)=200 Hz, the 45impedance measurements can be taken in less than half a second. Ofcourse, greater number of electrodes, electrode combinations 140, ormeasurement frequencies would increase this time. Although thefingerprints 180 are measured and stored in the log 210 periodically,such measurements need not occur at consistent intervals of time.

If it is not possible to fit a given impedance measurement into a givenquiet period—for example, when measuring impedance at lowerfrequencies—then some of the pulses may require delay or deletion, asshown in FIG. 10B. As shown, the quiet period has been delayed toaccommodate the time needed to take an impedance measurement, whichdepending on the measurement frequency and other timing consideration ofthe pulse train, may require simply delaying the issuance of a nextpulse slightly, or which may require deleting one or more pulses in thetrain. Because delaying or deleting pulses impacts patient therapy,impedance measurements may only be taken once during a measurementperiod 250, which should be long enough to allow many uninterruptedtherapy pulses 260 to occur at their specified frequency, f_(pulse). Fortypical timings, it is not expected that the delay periods in FIG. 10Bwould be significant enough that the patient would notice any change intherapy. Moreover, for typical timings the measurement period 250 willtypically be short enough to make the required numbers of measurements(45) in a reasonable time to update the fingerprint 180 in the log 210(again, at least every minute or so).

Various changes can be made to the particular implementation detailsdescribe herein. For example, a constant AC voltage source could be usedin lieu of constant AC current source 105, in which case the resultingcurrent through the tissue would be monitored to determine impedance.

While it was assumed that multiple measurement signals at differentfrequencies be applied between any two electrodes to determine thecomponent values in the tissue model, this is not strictly necessary.For example, a single measurement signalI(t)=|I₁|*e^(j2πf2t)+|I₃|*e^(j2πf3t) could be used that comprises threefrequency components, f1, f2, and f3. This would allow the impedancealgorithm to determine the complex impedance between any electrodecombination, and thus the values of the components in the tissue modelbetween them, using a single measurement signal. In this regard, it isnot strictly necessary that the measurement signal be comprised of onlythe frequencies necessary to determine the complex impedance: anyperiodic signal from a Fourier transform standpoint providing a suitablenumber of frequencies will suffice, even if such measurement signalcontains other significant frequencies, which may be filtered out ifthey are unnecessary. Provision of a suitable singular measurementsignal comprising the requisite number of frequencies would require amore sophisticated current source 105, but such is within the skill ofone in the art.

While, while the disclosed technique is particularly applicable to IPGsused in SCS applications, the technique is not so limited, and can beused with any multi-electrode implantable neurostimulation used in otherportions of a patient's body for any purpose. In one additional example,the disclosed technique can be used with well-known Deep BrainStimulators (DBS) as well.

Although particular embodiments of the present invention have been shownand described, it should be understood that the above discussion is notintended to limit the present invention to these embodiments. It will beobvious to those skilled in the art that various changes andmodifications may be made without departing from the spirit and scope ofthe present invention. Thus, the present invention is intended to coveralternatives, modifications, and equivalents that may fall within thespirit and scope of the present invention as defined by the claims.

What is claimed is:
 1. An implantable stimulator device, comprising: aplurality of electrodes configured to provide electrical stimulation toa patient; and impedance monitoring circuitry configured to apply atleast one AC measurement signal to a plurality of combinations of theelectrodes and to determine an impedance between each of the pluralityof combinations of the electrodes using the at least one AC measurementsignal.
 2. The device of claim 1, wherein the determined impedances arecomplex impedances.
 3. The device of claim 1, wherein the impedancemonitoring circuitry is configured to apply a plurality of ACmeasurement signals to each of the plurality of combination ofelectrodes.
 4. The device of claim 3, wherein the plurality of ACmeasurement signals are each of a different frequency, and wherein theimpedance between each of the plurality of combination of the electrodesis determined using the AC measurement signals at each of the differentfrequencies.
 5. The device of claim 1, wherein the impedance monitoringcircuitry is further configured to set a stimulation program to beprovided to the electrodes in accordance with the determined impedances.6. The device of claim 5, wherein the determined impedances areindicative of a posture of the patient.
 7. The device of claim 1,further comprising a database configured to store a plurality ofstimulation programs, wherein the determined impedances are used toselect a stimulation program to be provided to the electrodes from thedatabase.
 8. The device of claim 7, wherein the plurality of stimulationprograms are stored in the database during a fitting procedure.
 9. Thedevice of claim 7, wherein the stimulation programs are learned aselectrical stimulation is provided to the patient.
 10. The device ofclaim 7, wherein the selected stimulation program comprises one or morestimulation parameters, including which electrodes are active, apolarity of the active electrodes, a magnitude of the electricalstimulation at the active electrodes, a duration of electricalstimulation, and a frequency of the electrical stimulation.
 11. Thedevice of claim 1, wherein the impedance monitoring circuitry isconfigured to determine an impedance between each of the plurality ofcombinations of the electrodes by assessing responses to the at leastone AC measurement signal at each of the plurality of combination ofelectrodes.
 12. The device of claim 11, wherein the at least onemeasurement signal comprises an AC current, and the responses compriseAC voltages.
 13. The device of claim 1, wherein the impedance monitoringcircuitry is configured to apply the at least one AC measurement signalduring quiet periods when electrical stimulation is not occurring at anyof the plurality of electrodes.
 14. The device of claim 1, wherein theimpedance monitoring circuitry is further configured to use theimpedance determined between each of the plurality of combination ofelectrodes are to derive values of electrical components in a tissuemodel.
 15. The device of claim 1, further comprising a conductive case,wherein one of the plurality of electrodes comprises the case.
 16. Thedevice of claim 1, further comprising at least one lead, wherein theplurality of electrodes are positioned on the at least one lead.
 17. Animplantable stimulator device, comprising: a plurality of electrodesconfigured to provide electrical stimulation to a patient; and impedancemonitoring circuitry configured to generate a plurality of ACmeasurement signals each of a different frequency, apply each of theplurality of AC measurement signals to a plurality of combinations ofthe electrodes, determine an impedance between each of the plurality ofcombinations of the electrodes using each of the plurality of ACmeasurement signals, and derive values of electrical components in atissue model using the impedances determined between each of theplurality of combination of electrodes.
 18. The device of claim 17,wherein the impedance monitoring circuitry is further configured to seta stimulation program to be provided to the electrodes in accordancewith the determined impedances.
 19. The device of claim 18, wherein theselected stimulation program comprises one or more stimulationparameters, including which electrodes are active, a polarity of theactive electrodes, a magnitude of the electrical stimulation at theactive electrodes, a duration of electrical stimulation, and a frequencyof the electrical stimulation.
 20. The device of claim 17, furthercomprising at least one lead, wherein the plurality of electrodes arepositioned on the at least one lead.